Although small molecules remain the major drugs used in clinic, in numerous cases, their therapeutic impact has reached limitations such as insufficient capability to reach targets, lack of specificity, requirement for high doses leading to toxicity and major side effects. Over the past ten years, in order to circumvent limitations of small molecules and of gene-based therapies, we have witnessed a dramatic acceleration in the discovery of larger therapeutic molecules such as proteins, peptides and nucleic acids which present a high specificity for their target but do not follow Lipinski's rules. Pharmaceutical potency of these molecules remains restricted by their poor stability in vivo and by their low uptake in cells. Therefore, “delivery” has become a central piece of the therapeutic puzzle and new milestones have been established to validate delivery strategies: (a) lack of toxicity, (b) efficiency at low doses in vivo, (c) easy to handle for therapeutic applications (d) rapid endosomal release and (e) ability to reach the target. Although viral delivery strategies had given much hope for gene and cellular therapies, their clinical application has suffered from side—and toxicity—effects [1,2]. Researches were mainly focused on the development of non-viral strategies, and different methods have been proposed including lipid, polycationic nanoparticles and peptide-based formulations, but only few of these technologies have been efficient in vivo and have reached the clinic. Cell Penetrating Peptides (CPP) are one of the most promising non-viral strategies. Although definition of CPPs is constantly evolving, they are generally described as short peptides of less than 30 amino acids either derived from proteins or from chimeric sequences. They are usually amphipathic and possess a net positive charge [3-5]. CPPs are able to penetrate biological membranes, to trigger the movement of various biomolecules across cell membranes into the cytoplasm and to improve their intracellular routing, thereby facilitating interactions with the target. CPPs can be subdivided into two main classes, the first requiring chemical linkage with the cargo and the second involving the formation of stable, non-covalent complexes. CPPs from both strategies have been reported to favour the delivery of a large panel of cargos (plasmid DNA, oligonucleotide, siRNA, PNA, protein, peptide, liposome, nanoparticle . . . ) into a wide variety of cell types and in vivo models [3-7].
Twenty years ago, the concept of protein transduction domain (PTD) was proposed based on the observation that some proteins, mainly transcription factors, could shuttle within cells and from one cell to another [for review see ref 3,4]. The first observation was made in 1988, by Frankel and Pabo. They showed that the transcription-transactivating (Tat) protein of HIV-1 could enter cells and translocate into the nucleus. In 1991, the group of Prochiantz reached the same conclusions with the Drosophila Antennapedia homeodomain and demonstrated that this domain was internalized by neuronal cells. These works were at the origin of the discovery in 1994 of the first Protein Transduction Domain: a 16 mer-peptide derived from the third helix of the homeodomain of Antennapedia named Penetratin. In 1997, the group of Lebleu identified the minimal sequence of Tat required for cellular uptake and the first proofs-of-concept of the application of PTD in vivo, were reported by the group of Dowdy, for the delivery of small peptides and large proteins. Historically, the notion of Cell Penetrating Peptide (CPP) was introduced by the group of Langel, in 1998, with the design of the first chimeric peptide carrier, the Transportan, which derived from the N-terminal fragment of the neuropeptide galanin, linked to mastoparan, a wasp venom peptide. Transportan has been originally reported to improve the delivery of PNAs both in cultured cells and in vivo. In 1997, the group of Heitz and Divita proposed a new strategy involving CPP in the formation of stable but non-covalent complexes with their cargo [7]. The strategy was first based on the short peptide carrier (MPG) consisting of two domains: a hydrophilic (polar) domain and a hydrophobic (apolar) domain. MPG was designed for the delivery of nucleic acids [7]. The primary amphipathic peptide Pep-1 was then proposed for non-covalent delivery of proteins and peptides [8]. Then the groups of Wender and of Futaki demonstrated that polyarginine sequences (Arg8) are sufficient to drive small and large molecules into cells and in vivo. Ever since, many CPPs derived from natural or unnatural sequences have been identified and the list is constantly increasing. Peptides have been derived from VP22 protein of Herpes Simplex Virus, from calcitonin, from antimicrobial or toxin peptides, from proteins involved in cell cycle regulation, as well as from polyproline-rich peptides [reviews 4-6].